Typical examples of a dopa include dopa [.beta.-(3,4-dihydroxyphenyl)-.alpha.-alanine] and methyldopa [L-3-(3,4-dihydroxyphenyl)-2-methylalanine]. The former is known as a remedy for Parkinson's disease and the latter is known as a hypotensive agent. These compounds, however, have poor solubility in water and are inconvenient to be used in the form of a solution, particularly an injection. For example, the solubility of dopa in water is 2.5 mg/ml and 20 ml of water is necessary to dissolve the minimal dose (50 mg of dopa). Such circumstances require large ampules which are inconvenient for use. Therefore, preparations containing these compounds at a high concentration have been desired. Heretofore, there have been known attempts for the preparation of a highly concentrated dopa preparation; U.S. Pat. Nos. 3,916,004 and 3,911,137. The former comprises reacting a dopa with a non-toxic polyvalent metal salt to form a complex and the latter comprises adding a high molecular weight compound to a dopa to dissolve the dopa at a high concentration. However, the concentration of the dissolved dopa in these two inventions has not yet been sufficient to meet the requirement in the pharmaceutical field and there has been a demand for a process for preparing a much more highly concentrated dopa preparation. This invention provides a process for preparing a far more highly concentrated dopa preparation than made in a prior art. Also, another object of this invention is to provide a highly concentrated preparation of a dopa compound in the form of an aqueous solution or a freeze-dried preparation thereof.